1. Technical Field
The present invention relates to a lighting apparatus constructed to change the light quantity of a light source on the basis of an externally supplied control signal.
2. Description of Related Art
In lighting apparatuses used for indoor/outdoor lighting, an incandescent lamp, a fluorescent lamp or the like is conventionally used as a light source. Since the brightness of a light emitting diode (hereinafter referred to as an LED) has been improved and a blue LED has been developed recently, a white LED obtained by combining a blue LED and a phosphor has been put to practical use, and accordingly, an LED having characteristics of compactness, small power consumption, a long life and the like is used as a light source for the lighting.
With respect to lighting apparatuses using conventional light sources, a lighting apparatus having a structure in which the light source may be dimmed in accordance with a control signal such as a dimming control signal supplied from external equipment such as a dimming level controller is widely used. As this control signal, a PWM (pulse width modulation) signal having a duty ratio varied in accordance with a dimming level (i.e., a level of brightness) is generally used. Also with respect to lighting apparatuses using LEDs as light sources, various lighting apparatuses having a structure in which the light source may be similarly dimmed in accordance with such a control signal have been proposed (see, for example, Japanese Patent Application Laid-Open No. 2003-157986).
A lighting apparatus disclosed in Japanese Patent Application Laid-Open No. 2003-157986 is constructed to perform dimming level control, on the basis of a pulse signal externally inputted as a control signal, by combining control of a voltage to be applied to an LED and switch dimming with a switching element for connecting/disconnecting an LED current.
In this lighting apparatus, the voltage to be applied is controlled on the basis of the control signal by varying the on time of the on/off operation of the switching element with the voltage to be applied to the LED kept constant when the extent of dimming is small (i.e., when it is bright), and by varying the voltage to be applied to the LED with the on time of the on/off operation of the switching element kept constant when the extent of dimming is large (i.e., when it is dark). In this manner, the dimming level control may be performed in proportion to the control signal even when the extent of dimming is large (i.e., when it is dark).